[X86][SSE] combineReductionToHorizontal - add vXi8 ISD::MUL reduction handling (PR39709)

Default expansion leads to repeated extensions/truncations to/from vXi16 which shuffle combining and demanded elts can't completely unravel.

Better just to promote (any_extend) the input and perform a vXi16 reduction.

We'll be able to remove a lot of this if we ever get decent legalization support for reduction intrinsics in SelectionDAG.